A constant warning time device (often referred to as a crossing predictor or a grade crossing predictor in the U.S., or a level crossing predictor in the U.K.) is an electronic device that is connected to the rails of a railroad track and is configured to detect the presence of an approaching train and determine its speed and distance from a crossing (i.e., a location at which the tracks cross a road, sidewalk or other surface used by ironing objects). The constant warning time device will use this information to generate a constant warning time signal for controlling a crossing warning device. A crossing warning device is a device that warns of the approach of a train at a crossing, examples of which include crossing gate arms (e.g., the familiar black and white striped wooden arms often found at highway grade crossings to warn motorists of an approaching train), crossing lights (such as the red flashing lights often found at highway grade crossings in conjunction with the crossing gate arms discussed above), and/or crossing bells or other audio alarm devices. Constant warning time devices are often (but not always) configured to activate the crossing warning device at a fixed time (e.g., 30 seconds) prior to an approaching train arriving at a crossing.
Typical constant warning time devices include a transmitter that transmits a signal over a circuit formed by the track's rails and one or more termination shunts positioned at desired approach distances from the transmitter, a receiver that detects one or more resulting signal characteristics, and a logic circuit such as a microprocessor or hardwired logic that detects the presence of a train and determines its speed and distance from the crossing. The approach distance depends on the maximum allowable speed of a train, the desired warning time, and a safety factor. Preferred embodiments of constant warning time devices generate and transmit a constant current AC signal on said track circuit; constant warning time devices detect a train and determine its distance and speed by measuring impedance changes caused by the train's wheels and axles acting as a shunt across the rails, which effectively shortens the length (and hence lowers the impedance) of the rails in the circuit. Multiple constant warning devices can monitor a given track circuit if each device measures track impedance at a different frequency. Measurement frequencies are chosen such that they have a low probability of interfering with each other while also avoiding power line harmonics.
Federal regulations mandate that a constant warning time device be capable of detecting the presence of a train as it approaches a crossing and to activate the crossing warning devices in a timely manner that is suitable for the train speed and its distance from the crossing. In addition, the device must be capable of detecting trains that approach the crossing from both sides of the crossing (e.g., from east to west and from west to east, north to south and south to north, etc.).
One way to achieve this is to use two uni-directional track circuits, one that detects the presence of the train approaching from a first direction and one that detects the presence of the train approaching from a second direction. Uni-directional track circuits often employ insulated track joints. An insulated track joint requires the rails to be physically cut. Since the rails on either side of these cuts are required to be aligned to prevent derailment and other problems, insulated track joints require additional maintenance and monitoring, which is undesirable.
Although bi-directional track circuits can detect the direction of approaching trains from both sides of the crossing, they often require extra signaling or calculations, which is also undesirable. Thus, there is a need and desire for a fast and reliable technique fir determining the direction of a train travelling along a railroad track.